4 Answers
4

Go to level 3-2. Play the level until you get to the section with 4 bullet bills with a green warp pipe in the middle. Keep hopping on the bullets shot at you without touching the ground (hint: utilize shaking the Wii remote to stay in the air slightly longer). At first you will only gain points, but eventually every bullet bill you hit will get you a 1-up. It might take a while to get the hang of, but it's worth it.

You can also gain 1-ups quickly in level 2-3. At the very end of the stage, you have to run as far right as you can on any stair. Then, jump so that you land on the Koopa that is falling down the stairs towards you. After that, don't press anything. This method is much easier (if you can time the jump properly) as you just have to let your character jump on the shell repeatedly. Here's a video of this one.

That last one is a throw back to the original Super Mario Bros. There was a specific level you could do that on in the original as well (World 3, iirc. I don't remember which level).
– whaleyAug 11 '10 at 22:08

Go to World 6 and get hit by one of the Bullet Bills on the map to go to that toad-saving minigame. Try to get all the toad balloons without touching any Bullet Bills. The more Bullet Bills are on screen at the end, the more lives you get. I usually get 2-3 lives per attempt and very rarely walk away with none. It's pretty simple and very easy to repeat and practice.

One of the levels that I frequently use to get lives on is World 3, the first fortress just after the checkpoint, you can just save the pow until all of the dry-bones have fallen and just before the spiked balls start killing, then let the pow go. I can usually get 7-11 lives using this method. This method works best if you are playing multiplayer, when you get the lives, you and whoever else you are playing with can bubble, you lose no lives and keep the checkpoint so you can rinse and repeat to 99 lives on all of the characters. Hope this helps you.
:)

I have always gone to World 1-3 with the Yoshis. (IMPORTANT Do not kill any enemies until you get to the fire pirana plant.) Once you get to the fire pirana, eat it, head past the Star Coin, turn left, duck, and shoot. Then, all that you have to do is escort the fireball back to the beginning, and every enemy that it kills after a certain point will give you a live. You can even maximize potential by spawning each enemy as quickly as possible, and throwing any Koopa shells onto the ground.